A Step-by-Step Guide to Using the NIOSH Lifting Equation for Multiple Tasks

by Mark Middlesworth | December 19

Editor’s note: This is another monster post! If you’d rather work your way through this guide with a PDF file, just scroll down to the bottom of the post under “NIOSH Lifting Equation Resources”. Also included in the resources section is the NIOSH equation calculator, data collection worksheets, additional multiple task examples and the original NIOSH Applications Manual.

Introduction to the NIOSH Lifting Equation for Multiple Tasks

The NIOSH Equation is a tool used by occupational health and safety professionals to assess the manual material handling risks associated with lifting and lowering tasks in the workplace. This equation considers job task variables to determine safe lifting practices and guidelines.

The primary product of the NIOSH equation is the Recommended Weight Limit(RWL), which defines the maximum acceptable weight (load) that nearly all healthy employees could lift over the course of an 8 hour shift without increasing the risk of musculoskeletal disorders (MSD) to the lower back.

In addition, a Lifting Index (LI) is calculated to provide a relative estimate of the level of physical stress and MSD risk associated with the manual lifting tasks evaluated.

For more information regarding the equation and its outputs, you can review our guide and examples for single task analysis here. The single task guide will show you how to measure task variables and record them into our data collection worksheet, and how to use our single task lifting calculator to determine the RWL and LI for a single lifting task.

The purpose of this Multi-Task Guide is to show you how to analyze multi-task manual lifting jobs using our multi-task data collection worksheet and Composite Lifting Index (CLI) calculator. The product of this calculator is the CLI, which represents the level of risk associated with the collective material handling demands of the entire job.

Determine Evaluation Method

The analyst must first determine if the job should be analyzed as a single-task or multi-task manual lifting job. This can be accomplished by interviewing and observing workers performing the job to gain a complete understanding of all required lifting tasks. Selection of the lifting tasks to be evaluated should be based on the most significant and demanding manual material handling tasks. If the job requires a wide variety of lifting tasks, a multi-task evaluation can be performed using a composite of all single-task lifting assessments performed.

A single-task manual lifting job is defined as a lifting job in which the task variables do not significantly vary from task to task, or only one task is of interest (e.g., worst case analysis). This may be the case if the effects of the other tasks on strength, localized muscle fatigue, or whole-body fatigue do not differ significantly from the worst case task.

On the other hand, multi-task manual lifting jobs, are defined as jobs in which there are significant differences in task variables between tasks, are more difficult to analyze because each task must be analyzed separately. A specialized procedure using the Composite Lifting Calculator to determine the CLI can be used to analyze multi-task manual lifting jobs.

Many of the lifting jobs in the workplace have multiple lifting activities, and therefore could be analyzed as either a single or a multi-task lifting job. When detailed information is needed to specify engineering modifications, the multi-task approach can be used. The evaluator should also recognize that the multi-task procedure is more complicated than the single-task procedure, and requires a greater understanding of assessment terminology and mathematical concepts of the equation.

Therefore, the decision to use the single or multi-task approach should be based on:

The need for detailed information about all facets of the multi-task lifting job,

The need for accuracy and completeness of data in performing the analysis, and

The analyst’s level of understanding of the assessment procedures.

If you have determined that you will use the composite lifting calculator to determine CLI for a multi-task job, you will first use the multi-task data collection worksheet to gather the needed information and measurements for lifting task variables, and then record the data which will be used to calculate the RWL and LI for each task.

How to Use the NIOSH Lifting Equation for Multiple Tasks

Step 1: Measure and Record Task Variables

Horizontal Location of the Hands (H) – Measure and record the horizontal location of the hands at both the start (origin) and end (destination) of the lifting task. The horizontal location is measured as the distance (inches) between the employee’s ankles to a point projected on the floor directly below the mid-point of the hands grasping the object as pictured below:

Vertical Location of the Hands (V) – Measure and record the vertical location of the hands above the floor at the start (origin) and end (destination) of the lifting task. The vertical location is measured from the floor to the vertical mid-point between the two hands as shown below. The middle knuckle can be used to define the mid-point.

Vertical Travel Distance (D) – The vertical travel distance of a lift is determined by subtracting the vertical location (V) at the start of the lift from the vertical location (V) at the end of the lift. For a lowering task, subtract the V location at the end from the V location at the start.

Asymmetric Angle (A) – Measure the degree to which the body is required to twist or turn during the lifting task. The asymmetric angle is the amount (in degrees) of trunk and shoulder rotation required by the lifting task. Note: Sometimes the twisting is not caused by the physical aspects of the job design, but rather by the employee using poor body mechanics. If this is the case, no twisting (0 degrees) is required by the job. If twisting is required by the design of the job, determine the number of degrees the back and body trunk must twist or rotate to accomplish the lift. (i.e. 90° as pictured below)

Coupling (C) – Determine the classification of the quality of the coupling between the worker’s hands and the object as good, fair, or poor (1, 2, or 3). A good coupling will reduce the maximum grasp forces required and increase the acceptable weight for lifting, while a poor coupling will generally require higher maximum grasp forces and decrease the acceptable weight for lifting.

Frequency (F) – Determine the appropriate lifting frequency of lifting tasks by using the average number of lifts per minute during an average 15 minute sampling period. For example, count the total number of lifts in a typical 15 minute period of time and divide that total number by 15.

Load (L) – Determine the weight of the object lifted. If necessary, use a scale to determine the exact weight. If the weight of the load varies from lift to lift, you should record the average and maximum weights lifted.

We have developed the following worksheet to assist you with multi-task data collection. Below is an example of a completed data collection worksheet with four lifting tasks:

Multi-Task Data Collection Worksheet:

Step 2: Perform Single-Task Evaluations

Next, all lifting tasks should be evaluated with the our single task calculator to determine the Recommended Weight Limit (RWL), Lifting Index (LI), Frequency Independent Recommended Weight Limit (FIRWL), and the Frequency Independent Lifting Index (FILI). For more information on using the single task calculator, see our single task guide: Click here.

Important things to remember: For lifting tasks that require significant control at the destination, evaluate both the origin and the destination of the lift. Significant control is required if (1) the worker has to re-grasp the load near the destination of the lift, (2) the worker has to momentarily hold the object at the destination, or (3) the worker has to position or guide the load at the destination. The purpose of evaluating both the origin and destination of the lift is to identify the most stressful location of the lift. Therefore, the lower of the RWL values at the origin or destination should be used to compute the Lifting Index for the task, since this value would represent the limiting set of conditions.

Using the single task lifting calculator, you should get the following results for the four tasks in the above example:

Step 3: Renumber Tasks

The tasks are now renumbered in order of decreasing physical stress, beginning with the task with the greatest Lifting Index (LI) value down to the task with the smallest LI value. The tasks are renumbered in this way so that the more difficult tasks are considered first by the CLI Calculator. Using the above four task example, the renumbering is outlined below.

Step 4: Calculate Composite Lifting Index (CLI)

After the tasks are renumbered (highest LI to lowest), the variables are entered into the CLI calculator in that order (left to right) to compute the CLI for the job. Our version of the composite lifting calculator using the example values is pictured below:

Note: The calculator uses the acronyms STRWL and STLI. This is only to clarify that these values represent the Single Task Recommended Weight Limit (STRWL) and the Single Task Lifting Index (STLI) for each lifting task.

The CLI value for this job is 1.63, the job design goal is 1.0 or below. This means that many healthy workers would find this job physically stressful.

Example –Multi-Task Lifting Analysis

Job Description:

This warehouse worker picks various sizes and weights of product from five different levels of a storage rack and places product on a cart for transport to a conveyor line. The average weight of the product lifted is 14-22 lb., and the maximum weight lifted is 30-50 lb. depending on the rack level. Carrying is minimized by keeping the cart as close as possible, and significant control of the object is not required at the destination (dropping product onto cart) of any lift.

Job Analysis:

Since the warehouse picking job consists of more than one distinct task and the task variables vary significantly, the multi-task lifting analysis procedure should be used. This job is divided into five distinct lifting tasks represented by the five levels (A,B,C,D,E) of lifting from the storage rack. Task numbering in data collection is arbitrary and the sequencing does not need to reflect the order in which the tasks are performed.

Step 1: Measure and Record Task Variables

Measure and record the task variables for each task using the multi-task data collection worksheet as outlined below.

Using the single task lifting calculator, you should get the following results for the five lifting tasks in the above example:

Step 3: Renumber Tasks

The tasks are now renumbered in order of decreasing physical stress, beginning with the task with the greatest Lifting Index (LI) value down to the task with the smallest LI value. The tasks are renumbered in this way so that the more difficult tasks are considered first by the CLI Calculator. Using the above four task example, the renumbering is outlined below:

Step 4: Calculate Composite Lifting Index (CLI)

After the tasks are renumbered (highest LI to lowest), the variables are entered into the CLI calculator to compute the CLI for the job. Our version of the composite lifting calculator using these values is outlined below:

Note: The calculator uses the acronyms STRWL and STLI. This is only to clarify that these values represent the Single Task Recommended Weight Limit (STRWL) and the Single Task Lifting Index (STLI) for each lifting task.

The CLI for this job is 2.05, which indicates that there is a significant level of physical stress associated with this job that places many workers at an increased injury risk. Engineering and/or administrative controls are recommended to reduce the CLI to 1.0 or below.

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About the Author

Mark Middlesworth is the Founder of Ergonomics Plus, and still enjoys educating and motivating “workplace athletes” toward better health each and every day. Get more ergonomics and injury prevention advice from Mark by signing up for free updates.

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